High speed semiconductor devices utilizing ballistic transmission of hot electrons injected into a base region have been proposed. Recently, a new type high speed semiconductor device utilizing a negative resistance caused by a resonant tunneling effect and the high-speed nature of a hot electron has been proposed and attracted attention. FIGS. 20(a) and 20(b) are a schematic cross section and an energy band diagram, respectively, illustrating a resonanttunneling hot electron transistor (hereinafter referred to as RHET) disclosed in, for example, Japanese Published Patent Application No. 62-203371. In FIG. 20(a), there are successively disposed on an n type GaAs collector layer 6, an i type Al.sub.y Ga.sub.1-y As collector barrier layer 5, an n type GaAs base layer 4, a lower intrinsic Al.sub.x Ga.sub.1-x As potential barrier layer 2a, an intrinsic GaAs well layer 3, an upper intrinsic Al.sub.x Ga.sub.1-x As potential barrier layer 2b, and an n.sup.+ type GaAs emitter layer 1. An emitter electrode 71, a base electrode 72, and a collector electrode 73 are disposed on the emitter layer 1, the base layer 4, and the rear surface of the collector layer 6, respectively. Reference numeral 8 designates a resonance energy level and reference numeral 9 designates the Fermi level. The well layer 3 is sandwiched between the two potential barriers 2a and 2b, and the resonance level 8 is in the well layer 3, whereby a resonant tunneling barrier on the emitter side is produced. The well layer 3 is about 6 nm thick and each of the potential barriers 2a and 2b is about 5 nm thick.
FIGS. 21(a) to 21(c) are energy band diagrams for explaining the operating principle of the RHET. In the figures, the same reference numerals as in FIGS. 20(a) and 20(b) designate the same or corresponding parts, and reference numeral 100 designates hot electrons. A constant voltage V.sub.EC is applied between the emitter layer 1 and the collector layer 6.
FIG. 21(a) shows an energy band diagram in a state where the voltage V.sub.BE between the emitter layer 1 and the base layer 4 is zero. In this state, the energy of the resonance level 8 in the well layer 3 is higher than the energy of the Fermi level 9 in the emitter layer 1, so that no resonant tunneling from the emitter layer 1 to the base layer 4 occurs and no current flows.
However, as shown in FIG. 21(b), when a base voltage is applied so that the energy level at the emitter side is equivalent to the resonance level 8 in the well layer 3 (V.sub.BE =V.sub.R), electrons are injected into the base region due to the resonant tunneling. These electrons, i.e., hot electrons 100, are ballistically transmitted through the base layer 4 and the collector barrier layer 5 to reach the collector layer 6. At this time, current flows between the emitter layer 1 and the collector layer 6, and the RHET is turned on.
Thereafter, as shown in FIG. 21(c), when the base voltage V.sub.BE is increased to lower the energy of the resonance level 8 lower than the edge of the conduction band of the emitter layer 1, the resonant tunneling disappears, and the RHET is turned off.
In this way, the voltage/current characteristic of the RHET exhibits differential negative resistance with a peak current value at the base voltage V.sub.BE =V.sub.R. When the RHET is used as a multivalued logic circuit, the ratio of the current density in the ON state to the current density in the OFF state, i.e., a P/V (peak/valley) ratio should be sufficiently high. In practical use, a P/V ratio of 20:1 is required. During operation, the collector barrier layer 5 sufficiently insulates the base layer 4 from the collector layer 6, whereby only unscattered hot electrons 100 reach the collector layer 6 with high efficiency. More specifically, if the collector barrier layer 5 is designed so that the energy thereof may be equivalent to the resonance level 8 in the state of FIG. 21(b), electrons, which have been scattered in the base layer 4 and have lost energy, do not pass through the collector barrier layer 5 and only the unscattered hot electrons 100 reach the collector layer 6. That is, the high-speed nature of the RHET is realized only when the hot electrons 100 are used for the operation of the transistor. Because of the above-described differential negative resistance characteristics and the high-speed nature, the RHET is expected to be used as a high-speed multivalued logic circuit or a high-speed oscillator.
FIG. 23(a) is an energy band diagram of a prior art heterojunction bipolar transistor (hereinafter referred to as HBT) described in "Very High Speed Compound Semiconductor Device", written by Masamichi Ohmori, published on 1986 by Baifu-kan. FIG. 23(b) is a schematic cross section of the HBT. In the figures, there are successively disposed on a GaAs substrate 30, an n type GaAs collector layer 33, a p type GaAs base layer 32, and an n type AlGaAs emitter layer 31. A collector electrode 93, a base electrode 92, and an emitter electrode 91 are disposed on the collector electrode 33, the base electrode 32, and the emitter electrode 91, respectively. In addition, reference numerals 150 and 151 designate holes and electrons, respectively.
In the HBT, since the emitter-base junction is a heterojunction and the emitter is a wide energy band gap emitter, reverse injection of minority carriers from the base to the emitter is reduced, so that the emitter injection efficiency is high, resulting in a high current gain. Even if the base conductivity is increased, the high current gain is maintained, so that the base resistance is reduced. Accordingly, the HBT is a transistor capable of high-power and high-speed operation with high current gain.
In the above-described prior art RHET, in order to utilize the high-speed nature of the hot electrons 100, the collector barrier layer 5 is designed so that the energy of the electrons may be equivalent to the resonance level 8 in the state shown in FIG. 21(b). Therefore, electrons reflected by the collector barrier layer 5 are not negligible, and the current density in the ON state is not sufficiently high.
This problem will be described in more detail with reference to FIG. 22. FIG. 22 illustrates the transmissivities of an electron wave at various energies, which electron wave strikes the collector barrier layer 5 having a rectangular energy band potential profile. The transmissivities are calculated by solving the Schroedinger equation self-consistently. In the calculation, the energy of the collector barrier layer 5 is 0.4 eV. As shown in FIG. 22, the transmissivity of the electron wave begins to rise where the energy of the incident electron wave is around 0.3 eV and then Gradually increases until the incident electron wave energy becomes 0.4 eV. When the incident electron wave energy reaches 0.4 eV which is the energy of the collector barrier layer 5, the transmissivity steeply increases and, thereafter, slowly approaches 1 with an increase in the incident electron wave energy, but it never reaches 1. This is caused by reflection of the electron wave at the collector barrier layer 5 due to a quantum mechanical effect of the incident electron wave. Thereby, the transmissivity at an energy of 0.4 eV is only 30%. On the other hand, it is possible to increase the transmissivity by reducing the effective energy of the collector barrier layer 5. In this case, however, electrons which have been scattered in the base layer and have lost energy unfavorably pass through the collector barrier layer 5, whereby the RHET loses its high-speed nature. For the reasons described above, when only the hot electrons 100 are used in the transistor operation to achieve a high-speed operation, the current density in the ON state cannot be increased, so that a sufficient P/V ratio in practical use is not obtained. If the current density is increased, the RHET loses its high-speed nature.
In addition, while the prior art HBT including the wide band gap emitter achieves a high current gain as described above, if the reverse injection from the base is further reduced, a higher current gain is obtained.